BE setup with SQLserver - comments don't work

I recently developed a site incorporating BE and using the default XML setup for data storage. I then decided that before launching the site I would move over to SQL server.

Simply enough I followed the instructions from the setup folder, creating a DB using the SQL script provided and swapped out the web.config files. Everything seems to work great e.g. creating posts and users etc but whenever I try and post a comment I get
a pop up message saying:

"Sorry, the following error occurred while processing your comment:There was an error in the callback"

Note: I have not added any spam filters or other extensions besides the default ones. And the comment preview works fine too!